Download
1 / 30
310 likes | 522 Vues
VaR Methods. VaR. Portfolio theory: risk should be measure at the level of the portfolio not single asset Financial risk management before 1990 was measured at a level of a desk equity risk, currency risk, etc. all measured separately!
E N D
VaR • Portfolio theory: risk should be measure at the level of the portfolio not single asset • Financial risk management before 1990 was measured at a level of a desk equity risk, currency risk, etc. all measured separately! • VaR solves this problem it considers all sources of risk • VaR was born from the recognition that we need an estimate that accounts for various sources of risk and expresses loss in terms of probability Bahattin buyuksahin, Celso brunetti
VaR: Local Valuation • Local valuation methods make use of the valuation of the instruments at the current point first and (perhaps) second partial derivatives Example: Bonds (Worst d P) = (- D*P) Worst (d y) VaR(d P) = (D*P) VaR(d y) • Local valuation: because it uses information about the initial price and the exposure at the initial point • The distribution of the price is the same as that of the change in yield This is particularly convenient for portfolios with numerous sources of risks, because linear combinations of normal distributions are normally distributed Bahattin buyuksahin, Celso brunetti
VaR: Full Valuation • Full valuation methods: reprice the instruments over a broad range of values for the risk factors • Previous example: reprice the bond under different scenarios for the yield (Worst d P) = P[y0 + (Worst dy)] – P[y0] Bahattin buyuksahin, Celso brunetti
VaR: Delta-Gamma Method • Ideally, we would like to keep the simplicity of the local valuation while accounting for nonlinearities in the payoffs patterns Taylor Expansion • Note that the valuation is still local because we value the bond only once, at the original point first and second derivatives are also evaluated at the local point Bahattin buyuksahin, Celso brunetti
VaR: Delta-Gamma Method (Derivatives) • For a long call option, the worst value is achieved as the underlying price moves down by VaR(dS). With > 0 and > 0, the VaR for the derivative is • Delta-gamma it provides an analytical, second-order correction to the delta-normal VaR This explains why long positions in options, with positive gamma, have less risk than with a linear model Conversely, short positions in options have greater risk than implied by a linear model Bahattin buyuksahin, Celso brunetti
VaR: Delta-Gamma Method • This simple adjustment, unfortunately, works only when the payoff function is Monotonous it involves a one-to-one relationship between the option value f and S • More generally, the delta-gamma VaR method involves • Computing the moments of d f ( see Equation (15.7)) • Choosing the normal distribution that provides the best fit to these moments • The improvement brought about by this method depends on the size of the second order coefficient, as well as the size of the worst move in the risk factor • For forward contracts = 0, and there is no point in adding second-order terms. Similarly, for most fixed-income instruments over a short horizon, the convexity effect is relatively small and can be ignored Bahattin buyuksahin, Celso brunetti
Mapping The portfolio could consist of a large number of instruments, say M It would be too complex to model each instrument separately Mapping, replacing the instruments by positions on a limited number of risk factors Bahattin buyuksahin, Celso brunetti
Delta-Normal Method • It is a local valuation method • Simplest VaR approach: It assumes that the portfolio exposures are linear and that the risk factors are jointly normally distributed overall portfolio is normally distributed t+1 is the forecast of the variance-covariance matrix Bahattin buyuksahin, Celso brunetti
Delta-Normal Method • If the portfolio volatility is measured in dollars, VaR is equal to • This is called the diversified VaR, because it accounts for diversification effects. In contrast, the undiversified VaR is simply the sum of the individual VaRs for each risk factor • It assumes that all prices will move in the worst direction simultaneously, • which is unrealistic • The RiskMetrics approach is basically similar to the delta-normal approach the risk factor returns are measured as logarithms of the price ratios, instead of rates of returns • Main benefit of this approach: its simplicity • Drawbacks: The delta-normal method cannot account for nonlinear effects such as those encountered with options. It may also underestimate the occurrence of large observations because of its reliance on a normal distribution Bahattin buyuksahin, Celso brunetti
Historical Simulation Method • Full Valuation method • It uses historical distribution assumption: historical (past) distribution good representation of future • Define the current time as t The current portfolio value is Pt, which is a function of the current risk factors: Pt = P[f1,t , f2,t , f3,t , …, fN,t] • Sample the factor movements from the historical distribution, without replacement Bahattin buyuksahin, Celso brunetti
Historical Simulation Method • Construct hypothetical factor values, starting from the current one which are used to construct a hypothetical value of the current portfolio under the new scenario, using Equation (15.11): • We sort the returns and pick the one that corresponds to the -quantile • VaR is obtained from the difference between the average and the quantile Bahattin buyuksahin, Celso brunetti
Historical Simulation Method • The advantage of this method is that it makes no distributional assumption about the return distribution which may include fat tails • The main drawback of the method is its reliance on a short historical moving window to infer movements in market prices • If this window does not contain some market moves that are likely, it may miss some risks Bahattin buyuksahin, Celso brunetti
Monte Carlo Simulation Method • The Monte Carlo simulation method is basically similar to historical simulation, except that the movements in risk factors are generated by drawings from some distribution • This method is the most flexible, but also carries an enormous computational burden It requires users to make assumptions about the stochastic process and to understand the sensitivity of the results to these assumptions it is subject to model risk • Monte Carlo methods also create inherent sampling variability because of the randomization • Different random numbers will lead to different results • Note: when all risk factors have a normal distribution and exposures are linear, the method should converge to the VaR produced by the delta-normal VaR Bahattin buyuksahin, Celso brunetti
VaR Example • Evaluate the one-day downside risk of a currency forward contract • Compute VaR: • Value the portfolio • Mapping the value of the portfolio on fundamental risk factors • Generate movements in these risk factors • Combine the risk factors with the valuation model to simulate movements in the contract value • Assume that on December 31, 1998, we have a forward contract to buy £10 million in exchange for delivering $16.5 million in three months Bahattin buyuksahin, Celso brunetti
VaR Example • The current value of the forward contract is • There are three risk factors: spot exchange rate, domestic interest rate and foreign interest rate. After differentiation, we have • The forward contract is equivalent to A long position of (SP*) on the spot rate A long position of (SP*) on the foreign bill A short position of (KP) in the domestic bill (borrowing) Bahattin buyuksahin, Celso brunetti
VaR Example Q is quantity and the current value of the contract is given by St = 1.6637, rt = 4.9375%, rt* = 5.9688% and Pt* = 0.9854 Therefore Bahattin buyuksahin, Celso brunetti
Risk Factors • We need to convert the historical quotes of the risk factors into “random variables” zero mean and constant variance dr1 = 5.5625 – 5.5938 = - 0.0313 dS / S1 = (1.6315 – 1.6341) / 1.6341) = -0.0016 Bahattin buyuksahin, Celso brunetti
VaR: Historical Simulation The historical-simulation method takes historical movements in the risk factors to simulate potential future movements r1 = r0 + dr1 = 4.9375 – 0.0313 r*1 = r*0 + dr*1 = 5.9688 – 0.1563 S(1) = 1.6637 (1 – 0.0016) = 1.6611 We can then reprice the forward contract: Vt = £10,000,000 1.6611 0.9849 - $16,500,000 0.9879 = $59,941 Bahattin buyuksahin, Celso brunetti
VaR: Historical Simulation Repeat this process for all the movements from day 1 to day 100 This creates a distribution of contract values The final step consists of sorting the contract values Suppose we want to report VaR relative to the initial value (instead of relative to the average on the target date) The purpose of VaR is to report a single number as a downside risk measure 95% lower quantile VaRHS = $127,232 Bahattin buyuksahin, Celso brunetti
VaR: Delta-Normal Method • The delta-normal method takes a different approach to constructing the distribution of the portfolio value. We assume that the three risk factors (dS/S), (dP/P), (dP*/P*) are jointly normally distributed where the dz are normal variables and x are exposures • We compute VaR from 2(df) = x’ x Bahattin buyuksahin, Celso brunetti
VaR: Delta-Normal Method (df) = $77,306 VaRDN = 1.645 $77,306 = $127,168 This is very close to the VaR from historical simulation VaRHS = $127,232 Bahattin buyuksahin, Celso brunetti
